A quick example of how abstractions allow runtime substitution

dynamic-substitution-abstractions.cs

public class Program{
  static void Main(string[] args){
    ILogger logger;
    if(IsUserBirthday())
      logger = new BirthdayConsoleLogger();
    else
      logger = new ConsoleLogger();

    // SomethingThatUsesLoggers doesn't care if we're
    // logging birthday messages or not, it just
    // wants some kind of logger.
    var target = new SomethingThatUsesLoggers(logger);
  }
  
  bool IsUserBirthday(){
    // ... logic for determining birthday ...
    return true;
  }
}



public class SomethingThatUsesLoggers{
  public SomethingThatUsesLoggers(ILogger logger){
    logger.Log("Hello from SomethingThatUsesLoggers Constructor!");
  }
}

public interface ILogger{
  void Log(string message);
}

public class ConsoleLogger : ILogger {
  void Log(string message){
    Console.WriteLine(message);
  }
}

public class BirthdayConsoleLogger : ILogger {
  void Log(string message){
    Console.WriteLine("A Birthday Message For You:  {0}", message);
  }
}